1. Field of the Invention
The present invention concerns downhole/borehole gravity meters that sense variation in gravitational fields. More particularly, the present invention is directed to a dual-axis, tilt meter employed in subsurface oil exploration and retrieval.
2. Background Art
Gravity meters have been employed to measure characteristics of geologic formation and are used in the exploitation of hydrocarbon reservoirs found in geologic formations, commonly referred to as oil exploration and retrieval. Specifically, exploitation of hydrocarbon reservoirs involves characterizing oil, gas, and/or water.
Characterization of oil and gas in a hydrocarbon reservoir can be monitored as a function of gravity by analyzing borehole and surface gravity data. To that end, borehole gravity data is used to map out the vertical distribution of oil and gas at a well and surface gravity data can be employed to characterize the area of distribution.
Typically, borehole gravity surveys involve measuring gravity at differing locations in a borehole, which typically correspond to different vertical distances from the surface. The difference in gravity (xcex94{overscore (g)}) and the difference in vertical distance (xcex94{overscore (z)}) between two successive locations yield sufficient information to determine the bulk density of an area of the geologic formation adjacent to the borehole. The information concerning bulk rock density is mapped to determine the vertical distribution of oil and gas as the reservoir is exploited.
As a result, gravity measurements are typically monitored in the microgal (106 cm/s2) or nano-g range to ensure useable data that provides an indication of untapped pockets of oil or gas in the aforementioned area. This level of resolution in gravity measurements requires a highly precise gravity sensor and carefully implemented measuring techniques. For instance, the gravity sensor must be oriented so that the sensitive axis of the sensor is parallel to a vertical line representing the direction of gravity and commonly referred to as the plumb line.
To assist in properly aligning gravity sensors, many gravity meters include a tilt meter. The tilt meter is employed to minimize inclination of the gravity sensor greater than sensitive axis with respect to the plumb line. The tilt meter, however, must be properly aligned with respect to the gravity sensor for best results. Further, the relative alignment of the tilt meter and the gravity sensor should be checked periodically as the same may vary due to shock or vibration that occurs during field operations, particularly during transportation and handling. Another related issue is the sensitivity of the tilt meter, which may also change over time due to aging of the tilt meter and electronic components. In order to correct for these problems, the tilt meter is preferably calibrated prior to a gravity survey.
An exemplary gravity meter that includes the tilt meter/gravity sensor combination is available from SCINTREX(copyright) under the trade name CG-3. The CG-3 is a surface gravity meter, and the calibration method employed includes calibrating each tilt axis separately, with the axis orthogonal thereto remaining fixed The gain and offset of the calibration on each tilt axis are computed separately. The deviation from the plumb line is computed First, providing a calibrated offset value. The gain or sensitivity is then computed as a result of the calibrated offset value.
A drawback with the aforementioned calibration technique is that the same is difficult to employ in a subsurface gravity measurement tool, because calibrating the gravity tool based upon a fixed orthogonal axis introduces errors.
A need exists, therefore, to provide a method and a system to calibrate a gravity tool to provide accurate subsurface gravity measurements.
The invention provides a method for calibrating a subsurface gravity measurement device having a tilt meter and a gravity sensor. The method comprises associating tilt information produced by the gravity sensor as a function of tilt information produced by the tilt meter and an initial correction parameter; producing tilt data with the tilt meter, and gravity data, corresponding to the tilt data, with the gravity data being produced by the gravity sensor; fitting the tilt data and the gravity data to a polynomial equation, with the polynomial equation having a plurality of initial coefficients associated therewith, the initial coefficients including information concerning the initial correction parameter; and deriving a correction parameter as a function of the initial coefficients
The invention provides a method for calibrating, with respect to a plumb line, a gravity measurement device having a tilt meter and a gravity sensor. The method comprises associating tilt information produced by the gravity sensor as a function of a relationship between tilt information produced by the tilt meter and an initial correction parameter; orientating the tilt meter in a plurality of differing angular positions with respect to the plumb line, defining tilt data measuring, with the gravity sensor, gravity information at each of the angular positions defining gravity data; fitting the tilt data and the gravity data to a polynomial equation, with the polynomial equation having a plurality of initial coefficients associated therewith, the coefficients including information concerning the correction parameter; determining values for the plurality of initial coefficients using a least-squares regression; and deriving a correction parameter as a function of the coefficient values.
The invention provides a subsurface gravity measurement device, comprising a body: a tilt meter connected to the body to produce tilt data concerning angular positions the tilt meter forms with respect to a direction of gravity, with the direction of gravity defining a plumb line; a gravity sensor connected to measure the gravity and to produce information corresponding thereto, defining gravity data, with the information being a function of an angle the gravity measurement device forms with respect to the plumb line, defining tilt information, a processor in data communication with both the gravity sensor and the tilt meter; and a memory in data communication with the processor, the memory including a computer readable program to be operated on by the processor that includes a first subroutine to define a relationship between the tilt information produced by the gravity sensor and both the tilt data and an initial correction parameter, and a second subroutine to fit the tilt data and the gravity data to a polynomial equation, with the polynomial equation having a plurality of initial coefficients associated therewith, the initial coefficients including information concerning the correction parameter, and a third subroutine to derive a correction parameter as a function of the initial coefficients.